In seismic exploration, seismic data may be acquired by imparting acoustic energy into the Earth near its surface, and detecting acoustic energy that is reflected from boundaries between different layers of a subsurface rock formation. Acoustic energy is reflected when there is a difference in acoustic impedance between adjacent layers to a boundary. Signals representing the detected acoustic energy are interpreted to infer structures and composition of the subsurface rock formation structures.
In marine seismic exploration, a seismic energy source, such as an air gun, or air gun array, is typically used to impart the acoustic energy into the formations below the bottom of the water. The source is actuated at a selected depth in the water, typically while the source is being towed by a vessel. The same or a different vessel tows one or more seismic sensor cables, called “streamers”, in the water. Generally the streamer extends behind the vessel along the direction in which the streamer is towed. Typically, a streamer includes a plurality of receivers or sensors, such as hydrophones, for example, disposed on the cable at spaced apart, known positions along the cable. Hydrophones, as is known in the art, are sensors that generate an optical or electrical signal corresponding to the pressure of the water or the time gradient of pressure in the water. The vessel that tows the one or more streamers typically includes recording equipment to make a record, indexed with respect to time, of the signals generated by the receivers in response to the detected acoustic energy. The record of signals may be processed to infer structures of and compositions of the earth formations below the locations at which the seismic survey is performed.
Marine seismic data include effects that limit the accuracy of inferring the structure and composition of the subsurface rock formations. For example, effects relating to “ghost” reflections arise because water has a substantially different density and propagation velocity of pressure waves than the air above the water surface. Removal of these effects may be accomplished by source deghosting and receiver deghosting.
When a source is actuated, acoustic energy radiates generally outwardly from the source. Half of the energy travels downwardly where it passes through the water bottom and into the subsurface rock formations. The other half of the acoustic energy travels upwardly from the source and most of this energy reflects from the water surface, whereupon it travels downwardly. The reflected acoustic energy will be delayed in time and also shifted in phase from the directly downward propagating acoustic energy. This surface-reflected, downwardly traveling acoustic energy may be referred to as the “source ghost” signal.
When the acoustic energy propagates back up to the receivers after being reflected by the sea bottom and any subterranean formations, a portion of the energy propagates directly to the receivers. Another portion of the energy passes the level of the receivers and is reflected from the water surface back down to the receivers. The surface-reflected downward-propagating acoustic energy will be delayed in time and also shifted in phase from the directly upward propagating acoustic energy. This surface-reflected acoustic energy may be referred to as the “receiver ghost” signal.
It is highly desirable to improve techniques for the deghosting of marine seismic data.
While the invention will be described in connection with one or more embodiments, it will be understood that the invention is not limited to these. On the contrary, the invention is intended to cover all alternatives, modifications, and equivalents that may be included within the scope of the invention, as defined by the appended claims.